Many types of carrier recovery loops for demodulating an incoming signal are well-known in the art. In one type of conventional loop, known as a Costas type loop, a local oscillator is used to remove the carrier frequency from the incoming signal. Typically, the signal from the local oscillator is mixed with the incoming signal to produce one channel signal, and the local oscillator signal is phase shifted by 90.degree. and is mixed with the incoming signal to produce a second channel signal. The two channel signals are then applied to a feedback loop that locks the local oscillator in synchronism with the incoming carrier frequency. The mixing of this local oscillator signal and the incoming signal will remove the carrier frequency from the incoming signal. In this manner, the local oscillator is used to demodulate the incoming signal. It will be appreciated that the local oscillator in a Costas type loop must produce varying frequencies in order to follow the frequency drift of the incoming signal.
Because of the high data rate or frequency at which the Costas type loop must operate, it is not well suited for digital implementation. Therefore, a need has arisen for a carrier recovery loop for replacing a Costas type loop that is amenable to digital implementation. Specifically, a need has arisen for a recovery loop that may be implemented digitally without the problems associated with digitally implementing a recovery loop at high frequencies at which Costas loops are normally operated.
Analog demodulators often suffer from problems peculiar to analog devices such as D.C. offset and voltage controlled oscillator drift. A need has arisen for a digitally implemented recovery loop to overcome such problems. Furthermore, analog demodulators are not flexible for being adapted to varying data rates and different forms of demodulation. With a digital recovery loop, loop parameters may be changed by software modifications to allow operation at different data rates and forms of modulation such as phase shift keying and frequency shift keying. In addition, digital demodulation circuitry may be miniaturized using integrated circuits.